In many control applications digital signals are received from a part of an electrical or electronic circuit which have an effect on another part of that circuit to alter the ultimate output thereof. In some cases the received digital signals are of the kind known as "feedback" signals returned from a comparison device which compares the output of the circuit with a fixed reference.
One well known kind of circuit which utilizes feedback signals is a phase-lock loop in which an output signal generated by an oscillator is compared in phase and/or frequency with an input signal. A digital signal from the comparison device is then used to control the frequency of the oscillator. For example a digital-to-analogue converter may be used to convert the signal from the comparison device to an analogue voltage which is used to control a voltage controlled oscillation.
However, if a significant phase difference is present the time taken to adjust the output phase of the oscillator may be comparatively long. It would be advantageous to provide a rapid change to the control signal to effect a brief increase or decrease in the output oscillation to bring the output into phase alignment. When a digital-to-analogue converter is used to generate the controlling analogue system provision of such a change has required increasing the digital information rate which is not always easy to provide.
It is an object of the present invention to provide a simple control circuit capable of providing a digital control signal output the digital value of which may be changed more rapidly than was practical in the earlier systems outlined above.